Re: same query different execution plan (hash join vs. semi-hash join)

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От Huang, Suya
Тема Re: same query different execution plan (hash join vs. semi-hash join)
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Msg-id D83E55F5F4D99B4A9B4C4E259E6227CD0149AD18@AUX1EXC01.apac.experian.local
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Ответ на Re: same query different execution plan (hash join vs. semi-hash join)  (Tom Lane <tgl@sss.pgh.pa.us>)
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Thanks Tom, I think you're right. I just did an analyze on table test1 and the execution plan now generated is more
stableand predictable. 

Thanks,
Suya

-----Original Message-----
From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
Sent: Tuesday, May 20, 2014 12:22 AM
To: Huang, Suya
Cc: pgsql-performance@postgresql.org
Subject: Re: [PERFORM] same query different execution plan (hash join vs. semi-hash join)

"Huang, Suya" <Suya.Huang@au.experian.com> writes:
> Thank you Tom. But the time spent on scanning table test1 is less than 1 second (91.738 compares to 87.869), so I
guessthis shouldn't be the issue? 

No, the point is that the bad rowcount estimate (and, possibly, lack of stats about join column contents) causes the
plannerto pick a join method that's not ideal for this query. 

            regards, tom lane


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